The present invention relates generally to the monitoring or gauging of hydrostatic pressure, and more particularly to a compensation method for use in such systems to improve accuracy.
Various instruments are known for monitoring liquid levels in tanks, reservoirs and the like. One category of liquid monitoring instrument is responsive to changes of hydrostatic pressure at a submerged level in the liquid. In most cases, a so-called "bubbler system" is connected to the instrument, in which a small diameter tube runs from an instrument shelter above the water or liquid surface to a reference elevation near the bottom of the water. The lower end of the tube is, most often, secured slightly below the lowest anticipated liquid level. A trace flow of pressurized gas is passed down the tube, escaping as bubbles at the lower end. The pressure of gas at the lower end of the tube is equal to the hydrostatic pressure of the liquid at the submerged horizontal reference plane of the lower end of the tube. A suitable pressure responsive instrument or gauge is connected to measure the resultant gas pressure at a convenient elevation above the liquid. However, this pressure will not be identical to that at the lower end of the tube due to the vertical component of the weight of the gas in bubbler line. In fact, the pressure measured by the gauge (gauge pressure) will be lower than the hydrostatic pressure of the liquid at the submerged reference plane by an amount equal to the vertical component of the weight of the gas in the bubbler line. Also, for best accuracy, since the water or liquid filling the tank or reservoir displaces the atmosphere, compensation should be made for the vertical component of the displaced atmosphere, for the best accuracy. Other corrections, such as a correction for the local gravitational variation, may be necessary, dependent on the nature of the pressure responsive instrument itself.
A gas-weights compensation equation has been developed by the applicant in order to facilitate making these corrections in a pressure responsive instrument. This compensation has been employed for some time in "balanced beam manometer" types of pressure responsive instruments. Various instruments of the balanced beam manometer type are described in my U.S. Pat. Nos. 4,266,430, 4,274,039 and 4,277,981. In this type of instrument, gas weights and other compensations have been made by altering the calibration slope of the instrument slightly to cause it to read "high" by an amount equal to the applicable correction. This has been accomplished in the past by changing the poise weight, for example making the weight one percent low so that the instrument will read one percent high, and adding trim weights to the poise to adjust the calibration to a particular site.